Excessive osteoclast activity causes bone loss and is a significant health issue because it can lead to osteoporosis, tumor-associated bone disease, and other conditions characterized by reduced bone mass and strength. An antibody to the Lrp5/6 antagonist sclerostin is a promising new anabolic therapy because it enhances bone formation. Rodent and human studies indicate that this therapy also reduces osteoclast- mediated resorption. The mechanism(s) by which enhanced Wnt activity suppresses bone resorption is unknown. Our studies reveal that targeted deletion of Lrp5/6 in osteoclast precursors (pOC) reduces osteoblast numbers and bone formation rates, supporting a role for Wnt/Lrp in pOC recruitment of osteoblast lineage cells. This response is not seen with targeted deletion of ?-catenin in pOCs, indicating ?-catenin- independent impacts of Wnt/Lrp signaling on osteoclast lineage cells. The focus of this project is to investigate the mechanisms by which Wnt/Lrp influence pOC differentiation and recruitment of osteoblast lineage cells. Wnt3a suppresses pOC differentiation when it binds to Lrp5/6. However, when Lrp5/6 receptors are not present, Wnt3a enhances pOC differentiation. Our preliminary data reveal that in these instances, Wnt3a acts more like Wnt5a and signals through Ror2 to activate noncanonical Wnt signaling pathways. Mice lacking Ror2 in pOCs have osteopetrosis, with reduced osteoclast numbers. Surprisingly, this severe reduction in osteoclast lineage cells had no impact on osteoblast numbers, suggesting that Ror2 signaling suppresses osteoblast recruitment. Our preliminary data reveal that Wnt3a treatment of pOCs represses Rank expression and stimulates expression of the enzyme SPHK1, increasing production of its product, the chemokine S1P. In contrast, Wnt5a stimulates Rank expression and suppresses SPHK1/S1P production. The central hypothesis of this proposal is that there are two competing Wnt pathways that control osteoclast differentiation. The canonical arm is activated by Wnt3a, which signals through Lrp5/6, suppresses differentiation and promotes S1P production. The noncanonical arm is activated by Wnt5a and Ror2 to stimulate osteoclast differentiation and suppress S1P levels under normal conditions, but Ror2 is also engaged by Wnt 3a when Lrp5/6 levels are low. Our Specific Aims to test our hypothesis are to: (1) Determine the mechanisms of Lrp and Ror2 regulation of SPHK1 and Rank, (2) Examine Ror2 and SPHK1/S1P expression in murine and human osteoclast lineage cells, and (3) Evaluate osteoblast lineage cell recruitment in murine and human models of aging. These studies will significantly enhance our understanding of the mechanisms by which Wnts control bone metabolism and provide important new avenues to enhance bone anabolic influences of Wnt signaling while suppressing catabolic influences. These new therapy directions will improve strategies to prevent and treat osteoporosis and pathological bone loss in conditions such as arthritis and tumor-induced osteolysis.